FIG. 1 shows a prior art PVD chamber. The PVD chamber 104 includes a target 102, a chamber housing 106, a shield 108 and a pedestal 110 for holding an object to receive a physical vapor deposition of the target material. A magnetron 100 can be used to shape the plasma and the flow of ions to the target 102. A magnetron 100 can be one or more permanent magnets or electro-magnets of appropriate strength, orientation and position to achieve the desired shaping. In the preferred embodiment of the present invention, the target material is typically selected from among Aluminum, an Aluminum alloy, Titanium, Tungsten or a composite of Titanium and Tungsten. The object which receives a deposition of target material in the preferred embodiment is a semiconductor wafer 112. The chamber is evacuated of air and includes a predetermined partial pressure of a known gas, generally Argon.
The surface of the wafer 112 includes an opening 114 as shown in FIG. 2. As the density of devices on semiconductor wafers has increased and etching technology has evolved, the aspect ratio of openings on the surface of semiconductor wafers has correspondingly continually changed such that the depth of an opening can exceed the width as shown in FIG. 3. FIG. 3 shows an opening having an aspect ratio wherein the depth of the opening is approximately twice its width.
As is well known, during a deposition a plasma of gas such as is formed in the chamber. Ions from the plasma are attracted to the target by applying an appropriate DC voltage to the target. Usually the DC voltage is applied between the anode, e.g., the shield, at +DC and the cathode, e.g., the target, at -DC. The plasma can be formed by this same voltage. 500V is a common voltage selected for forming the plasma and accelerating the ions toward the target. As the plasma ions strike the target, particles of target material are sputtered from the surface of the target 102 at a significant kinetic energy. Because of the amount of kinetic energy imparted to the particles escaping from the target 102, the particles will typically adhere securely to any solid structure which they strike.
Particles of escaping target material can leave the surface of the target at any angle and from any surface location as shown by the arrows in FIG. 1. Accordingly, not only does the wafer 112 receive a deposited layer of the target material but also the inner walls of the shield are coated. In addition, particles of the target material can also strike the surface of a wafer 112 at almost any angle as shown by the arrows in FIG. 2.
FIG. 2 shows a surface geometry of a semiconductor wafer having an opening with a low aspect ratio (opening width to depth) receiving a PVD deposited coating of the target material. The deposited material will strike the wafer and the opening at a variety of angles from perpendicular to acute angles as shown in FIG. 2. Because of the geometries of the wafer, and, in particular, the low aspect ratio of the opening 114, there are no significant negative effects from the non-uniform deposition direction of the deposited material.
When depositing target material onto a wafer 112 having an opening 118 (FIG. 3) with a high aspect ratio, the deposited material striking the upper portions of the opening 118 at acute angles tends to close the mouth of an opening 118 before the entire opening 118 is completely filled such that a void 120 is formed within the opening as shown in FIG. 4. Such voids 120 can cause long term reliability failures in semiconductor devices.
As shown in FIG. 5, a collimator 122 is used to resolve such problems. A collimator 122 is a plate positioned between the target 102 and the semiconductor wafer 112. The collimator has a finite predetermined thickness and includes a number of passages 124 of predetermined dimensions formed through its thickness, through which the deposited material must pass on its path from the target 102 to the semiconductor wafer 112.
The collimator 122 filters out target material that would otherwise strike the wafer 112 at acute angles. The actual amount of filtering depends upon the aspect ratio of the passages 124 through the collimator 122, such that only deposited material within an angle of .THETA./2 from the perpendicular can pass through the collimator to strike the wafer 112. This allows improved semiconductor device manufacture for devices having openings 188 with high aspect ratios.
There are several disadvantages to the use of a collimator. First, each particle of target material which strikes the upper surface of the collimator plate is deposited on the collimator. This can significantly reduce the efficiency of depositing material in a PVD chamber because a portion of the target material is deposited on the collimator rather than the intended semiconductor wafer 112 and thus wastes target material.
Second, the deposited material can strike a side wall of a passage 124 at an angle greater than .THETA./2 from the perpendicular. Such material is thus deposited within and tends to clog the passages. This increases the aspect ratio of the passages 124 so that less material from the target will pass through the collimator and be deposited on the wafer 112 which will increase the throughput time of subsequent wafers which are processed at the same energy to achieve the same thickness of deposited material.
Third, as the thickness of the material deposited on the collimator increases, (especially with high stress material like TiW or TiN) the accumulated material on the collimator will flake-off and land on the wafer 112 which will likely destroy a circuit. Because of these reasons, the collimator must periodically be replaced to provide reasonable throughput and to avoid damaging circuits on the wafer 112.
What is needed is a collimator for use in a deposition chamber on which the target material does not build up.